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Search Results: 1 - 10 of 325481 matches for " S. Shahab Naghavi "
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A Critical Study of the Elastic Properties and Stability of Heusler Compounds: Phase Change and Tetragonal X2YZ Compounds  [PDF]
Shuchun Wu, S. Shahab Naghavi, Gerhard H. Fecher, Claudia Felser
Journal of Modern Physics (JMP) , 2018, DOI: 10.4236/jmp.2018.94050
Abstract: In the present work, the elastic constants and derived properties of tetragonal Heusler compounds were calculated using the high accuracy of the full-potential linearized augmented plane wave (FPLAPW) method. To find the criteria required for an accurate calculation, the consequences of increasing the numbers of k-points and plane waves on the convergence of the calculated elastic constants were explored. Once accurate elastic constants were calculated, elastic anisotropies, sound velocities, Debye temperatures, malleability, and other measurable physical properties were determined for the studied systems. The elastic properties suggested metallic bonding with intermediate malleability, between brittle and ductile, for the studied Heusler compounds. To address the effect of off-stoichiometry on the mechanical properties, the virtual crystal approximation (VCA) was used to calculate the elastic constants. The results indicated that an extreme correlation exists between the anisotropy ratio and the stoichiometry of the Heusler compounds, especially in the case of Ni2MnGa. Metastable cubic Ni2MnGa exhibits a very high anisotropy (28) and hypothetical cubic Rh2FeSn violates the Born-Huang stability criteria in the L21 structure. The bulk moduli of the investigated tetragonal compounds do not vary much (130 ...190 GPa). The averaged values of the other elastic moduli are also rather similar, however, rather large differences are found for the elastic anisotropies of the compounds. These are reflected in very different spatial distributions of Young’s moduli when comparing the different compounds. The slowness surfaces of the compounds also differ considerably even though the average sound velocities are in the same order of magnitude (3.2 ... 3.6 km/s). The results demonstrate the importance of the elastic properties not only for purely tetragonal Heusler compounds but also for phase change materials that exhibit magnetic shape memory or magnetocaloric effects.
Crystal structure search and electronic properties of alkali doped phenanthrene and picene
S. Shahab Naghavi,Erio Tosatti
Physics , 2014, DOI: 10.1103/PhysRevB.90.075143
Abstract: Alkali doped aromatic compounds have shown evidence of metallic and superconducting phases whose precise nature is still mysterious. In potassium and rubidium doped phenanthrene, superconducting temperatures around 5 K have been detected, but such basic elements as the stoichiometry, crystal structure, and electronic bands are still speculative. We seek to predict the crystal structure of M3-phenanthrene (M = K, Rb) using ab-initio evolutionary simulation in conjunction with density functional theory (DFT), and find metal but also insulator phases with distinct structures. The original P21 herringbone structure of the pristine molecular crystal is generally abandoned in favor of different packing and chemical motifs. The metallic phases are frankly ionic with three electrons acquired by each molecule. In the nonmagnetic insulating phases the alkalis coalesce reducing the donated charge from three to two per phenanthrene molecule. A similar search for K3-picene yields an old and a new structure, with unlike potassium positions and different electronic bands, but both metallic retaining the face-to-edge herringbone structure and the P21 symmetry of pristine picene. Both the new K3-picene and the best metallic M3-phenanthrene are further found to undergo a spontaneous transition from metal to antiferromagnetic insulator when spin polarization is allowed, a transition which is not necessarily real, but which underlines the necessity to include correlations beyond DFT. Features of the metallic phases that may be relevant to phonon-driven superconductivity are underlined.
The role of correlations in the high-pressure phase of FeSe
S. Shahab Naghavi,Stanislav Chadov,Claudia Felser
Physics , 2010, DOI: 10.1088/0953-8984/23/20/205601
Abstract: We present a systematic study of the high-pressure FeSe phase performed by means of the first-principle electronic structure calculations. Basing on available experimental information about the unit cell geometry we calculate the band structure and characterize the related properties during their pressure driven evolution. The electronic structure including the hybrid functional B3LYP or the Hubbard parameter U for the iron d states lead to the correct semiconducting ground state for the hexagonal stoichiometric FeSe within the broad pressure range (up to 30 GPa).
Robust $s\pm$ Superconductivity in a Two-Band Hubbard-Fr{?}hlich Model of Alkali Doped Organics
Tao Qin,Michele Fabrizio,S. Shahab Naghavi,Erio Tosatti
Physics , 2014, DOI: 10.1103/PhysRevB.90.064512
Abstract: The damaging effect of strong electron-electron repulsion on regular, electron-phonon %$s$-wave superconductivity is a standard tenet. In spite of that, an increasing number of compounds such as fullerides and more recently alkali-doped aromatics exhibit %$s$-wave or presumably $s$ wave superconductivity despite very narrow bands and very strong electron repulsion. Here, we explore superconducting solutions of a model Hamiltonian inspired by the electronic structure of alkali doped aromatics. The model is a two-site, two-narrow-band metal with a single intersite phonon, leading to attraction-mediated, two-order parameter superconductivity. On top of that, the model includes a repulsive on-site Hubbard $U$, whose effect on the superconductivity we study. Starting within mean field, we find that $s \pm$ superconductivity is the best solution surviving the presence of $U$, whose effect is canceled out by the opposite signs of the two order parameters. The correlated Gutzwiller study that follows is necessary because without electron correlations the superconducting state would in this model be superseded by an antiferromagnetic insulating state with lower energy. The Gutzwiller correlations lower the energy of the metallic state, with the consequence that the $s \pm$ superconducting state is stabilized and even strengthened for small Hubbard $U$.
Electron-doped organics: Charge-disproportionate insulators and Hubbard-Fr?hlich metals
S. Shahab Naghavi,Michele Fabrizio,Tao Qin,Erio Tosatti
Physics , 2013, DOI: 10.1103/PhysRevB.88.115106
Abstract: Several examples of metallic electron doped polycyclic aromatic hydrocarbons (PAHs) molecular crystals have recently been experimentally proposed. Some of them have superconducting components, but most other details are still unknown beginning with structure and the nature of metallicity. We carried out ab-initio density functional calculations for La-Phenanthrene (La-PA), here meant to represent a generic case of three-electron doping, to investigate structure and properties of a conceptually simple case. To our surprise we found first of all that the lowest energy state is not metallic but band insulating, with a disproportionation of two inequivalent PA molecular ions and a low P1 symmetry, questioning the common assumption that three electrons will automatically metallize a PAH crystal. Our best metallic structure is metastable and slightly higher in energy, and retains equivalent PA ions and a higher P21 symmetry -- the same generally claimed for metallic PAHs. We show that a "dimerizing" periodic distortion opens very effectively a gap in place of a symmetry related degeneracy of all P21 structures near the Fermi level, foreshadowing a possible role of that special intermolecular phonon in superconductivity of metallic PAHs. A Hubbard-Frohlich model describing that situation is formulated for future studies.
High pressure layered structure of carbon disulfide
S. Shahab Naghavi,Yanier Crespo,Roman Martonak,Erio Tosatti
Physics , 2015, DOI: 10.1103/PhysRevB.91.224108
Abstract: Solid CS$_{2}$ is superficially similar to CO$_{2}$, with the same $Cmca$ molecular crystal structure at low pressures, which has suggested similar phases also at high pressures. We carried out an extensive first principles evolutionary search in order to identify the zero temperature lowest enthalpy structures of CS$_{2}$ for increasing pressure up to 200\,GPa. Surprisingly, the molecular $Cmca$ phase does not evolve into $\beta$-cristobalite as in CO$_{2}$, but transforms instead into phases HP2 and HP1, both recently described in high pressure SiS$_{2}$. HP1 in particular, with a wide stability range, is a layered $P2_{1}/c$ structure characterized by pairs of edge-sharing tetrahedra, and theoretically more robust than all other CS$_{2}$ phases discussed so far. Its predicted Raman spectrum and pair correlation function agree with experiment better than those of $\beta$-cristobalite, and further differences are predicted between their respective IR spectra. The band gap of HP1-CS$_{2}$ is calculated to close under pressure yielding an insulator-metal transition near 50 GPa in agreement with experimental observations. However, the metallic density of states remains modest above this pressure, suggesting a different origin for the reported superconductivity.
Theoretical Study of New Acceptor and Donor Molecules based on Polycyclic Aromatic Hydrocarbons
S. Shahab Naghavi,Thomas Gruhn,Vajiheh Alijani,Gerhard H. Fecher,Claudia Felser,Katerina Medjanik,Dmytro Kutnyakhov,Sergej A. Nepijko,Gerd Sch?onhense,Ralph Rieger,Martin Baumgarten,Klaus M?ullen
Physics , 2010,
Abstract: Functionalized polcyclic aromatic hydrocarbons (PAHs) are an interesting class of molecules in which the electronic state of the graphene-like hydrocarbon part is tuned by the functional group. Searching for new types of donor and acceptor molecules, a set of new PAHs has recently been investigated experimentally using ultraviolet photoelectron spectroscopy (UPS). In this work, the electronic structure of the PAHs is studied numerically with the help of B3LYP hybrid density functionals. Using the DELTA-SCF method, electron binding energies have been determined which affirm, specify and complement the UPS data. Symmetry properties of molecular orbitals are analyzed for a categorization and an estimate of the related signal strength. While SIGMA-like orbitals are difficult to detect in UPS spectra of condensed film, calculation provides a detailed insight into the hidden parts of the electronic structure of donor and acceptor molecules. In addition, a diffuse basis set (6-311++G**) was used to calculate electron affinity and LUMO eigenvalues. The calculated electron affinity (EA) provides a classification of the donor/acceptor properties of the studied molecules. Coronene-hexaone shows a high EA, comparable to TCNQ, which is a well-known classical acceptor. Calculated HOMO-LUMO gaps using the related eigenvalues have a good agreement with the experimental lowest excitation energies. TD-DFT also accurately predicts the measured optical gap.
Cloud Computing as an Innovation in GIS & SDI: Methodologies, Services, Issues and Deployment Techniques  [PDF]
Mohammad Naghavi
Journal of Geographic Information System (JGIS) , 2012, DOI: 10.4236/jgis.2012.46062
Abstract: Cloud computing is one of the main issues of interest to the scientific community of the spatial data. A cloud is referred to computing infrastructure for a representation of network. From the perspective of providers, the main characteristics of cloud computing is being dynamic, high power in computing and storage. Also cloud computing is a cost benefit and effective way for representation of web-based spatial data and complex analysis. Furthermore, cloud computing is a way to facilitate distributed computing and store different data. One of the main features of cloud computing is ability in powerful computing and dynamic storage with an affordable expense and secure web. In this paper we further investigate the methodologies, services, issues and deployed techniques also, about situation of cloud computing in the past, present and future is probed and some issues concerning the security is expressed. Undoubtedly cloud computing is vital for spatial data infrastructure and consequently the cloud computing is able to expand the interactions for spatial data infrastructure in the future.


Effect of various parameters on the efficiency of zinc phosphate solubilization by indigenous bacterial isolates
S Shahab, N Ahmed
African Journal of Biotechnology , 2008,
Abstract: Zinc phosphate solubilization efficiency of ten soil bacteria were studied for various parameters like carbon sources, temperature, pH, variable concentration of sodium chloride and glucose. For majority of the isolates 20oC was appeared to be the optimum temperature for solubilization of zinc phosphate. Glucose was the most favorable carbon source for solubilization while lactose is the least favorable carbon source. pH 7 was the most favorable pH for solubilization while at pH 4 no growth and solubilization was seen. Except CMG859, no isolate solubilized at pH 8 and 9. CMG851 (Acinetobacter lwoffi) and CMG852 showed enhanced solubilization in presence of 1% sodium chloride. 1% glucose is required for the solubilization of zinc phosphate and no solubilization was appeared in presence of 0.1% glucose. CMG851 (A. lwoffi), CMG 860 (pseudomonas aeruginosa) CMG 857 (Bacillus thuringiensis) were found to be the most promising isolates.
Influence of antiferromagnetic fluctuations on the pressure dependence of the critical temperature in FeSe
S. Chadov,F. Casper,S. Naghavi,C. Felser
Physics , 2009,
Abstract: This paper has been withdrawn, due a wrong analysis of the obtained results.
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